WIP: Do not try to link to the JVM if JRE path is NULL

It makes no sense to set JAVA_HOME to the result of get_jre_home() if
that function returned null. In that case, we should instead simply fail
to create the Java VM.

Unfortunately, this change does not restore the behavior from before
eb1d0430ad01c681e032cb410335a42e9c7bbd4f -- on Mavericks, ImageJ still
launches with Java 7, whereas it used to launch with Java 6...